Variability of venation patterns in extant genus <em>Salix</em>: Implications for fossil taxonomy

Abstract

The extant genus Salix Linnaeus (1753) represents one of the most diverse groups of woody plants. Leaf areas vary from a few mm2 in arctic or high alpine habitats to more than 100 cm2 in humid subtropical zones. Salix leaves are represented across the range of possible leaf shapes, from circular, obovate, and ovate, to lanceolate and linear with a length-to-width ratio of up to 30. Leaf venation may be eucamptodromous, eucamptodromous with occasional brochidodromous or semicraspedodromous arches, or brochidodromous. Because brochidodromous and semicraspedodromous arches may occur on the same leaf, the more inclusive term brochoid is introduced here. This study gives an overview of venation patterns within extant genus Salix and sorts leaves into five venation-defined morphotype groups. In some species of subgenera Protitea and Salix, individuals in hot, dry environments develop long brochoid chains over most of the blade length or intramarginal veins with only tertiary-gauged connections to the secondary vein framework. These unusual venation patterns correlate with high mean monthly temperature (MMT) and low mean monthly precipitation (MMP) of the hottest month. This study also discusses possible reasons as to why intramarginal veins seem to be absent or at least rare in fossil Salix specimens. These findings aid in both distinguishing between fossil Salix species and in separating fossil Salix remains from those of other genera.